A machine tool comprising this type of tool head for machining pipe ends, in particular, for cutting threaded connectors, has been disclosed in DE 44 38 818 [U.S. Pat. No. 5,662,524]. The pipe thread is generally produced by a single pass of the tools over the tool end, and movements of the tools are numerically controlled. The aspect that is of special significance in these machines is the actual tool head in which the tools that produce the thread or another rotary machining (surfacing as well as outer and inner chamfering) of the pipe, are positioned against the securely clamped pipe. The pipe itself is moved to a predefined position of the machine, and the tool feed is switched on as soon as the pipe end has reached its exact position.
The machining tools are movable and distributed uniformly angularly of the tool head, where, although any number of tools may be used, the most frequent approach is to provide six tools, of which three each are moved in the same direction spaced 120° apart, while the other three are moved synchronously thereto in the opposite direction spaced 120° apart. This is effected by central gears that the toothed racks of the slides supporting the tools engage. The high rotational speeds of the tool head—for example, 1000 rpm—create very high centrifugal forces on the moving masses that act against the feed forces of the tools to be precisely positioned, which feed forces must be kept as low as possible. The gears engaging the slides, which are as spur gears, thus also is function simultaneously to compensate for centrifugal forces. However, total compensation of centrifugal forces is effected only when the slides occupy their center position, i.e. an intermediate adjustment position.
DE 101 33 856 discloses an approach whereby an additional compensation device is provided in order to effect compensation whenever the slides are no longer at the same radial distance from the axis of rotation due to the radial in-feed setting such that centrifugal forces acting on them are thus no longer of the same magnitude and can no longer balance out. This device is composed of a club-shaped counterweight including gear teeth that mesh with a gear engaged with a toothed rack of a first slide. However, the countertorque can only effect a relatively small deflection with the club-shaped counterweight. Greater differential centrifugal forces on the other slide that is not engaged with the counterweight can only by balanced by a counter-torque through the first slide and the central toothed gear mounted between the two slides. An alternative provides an approach wherein the counterweight is connected to the toothed gear, or is integrated in this gear.
The current systems of the NC thread-cutting head or tool head that are based on and operate according to this prior art are based on the following functional principle.                Three mutually independent pairs of slides are embedded in one base body of the tool head.        One slide each (A slide) is fitted with a spindle and bevel pinion.        The second slide (B slide) is connected to the A slide through a toothed segment (coupling gear) and toothed racks that are mounted along the sides of the slides.        A shared crown gear meshes with the bevel pinions, which gear is a component part of the main spindle and is connected through a gear stage to a main spindle drive and a differential gear.        The motion introduced into the differential gear produces radial movement of the A slides in the tool head.        Connecting the A slide to the B slide through the central gear or coupling gear enables the centrifugal forces acting on the slides to be partially compensated.        Due to an additional compensation weight inside the coupling gear and moved by a gear assembly, this compensation is amplified further up to the point of almost total compensation of centrifugal forces.        
The above-described construction of machines for machining pipe ends has inherent disadvantages in terms of precision and tolerances. This is due to the unavoidable tooth flank backlash between the coupling gear and the toothed rack that only allows a precision of at best plus/minus 0.09 mm to be achieved for the B slide—which in this example means a backlash of 0.18 mm when the slides are moved from inside to outside or visa versa. For this reason, the A slides that have a greater precision are used for cutting threads, while the B slides are used for the less demanding facing, and external and internal chamfering.
These tolerances are not adequate for producing high-specification premium couplings, which is understood to include special couplings in which conical threads, sealing seats, and abutment joint enhance the seal of the connection. This because these couplings require a precision in terms of thread diameter and sealing seat diameter that measures less than 0.01 mm, as well as a surface roughness on the sealing seat and in the abutment joint where Ra is less than or equal to 1.6 μm.